An economical and efficient method for separating triglycerides on the basis of degree of unsaturation has previously been sought to satisfy commercial pressures. For example, in Ou U.S. Pat. No. 4,961,881, the desirability of reducing the level of unsaturated fatty acid groups in synthetically produced triglycerides was disclosed since the product could be used as a cocoa butter extender. In view of more recent trends to reduce the monounsaturated components of triglyceride mixtures for health reasons, the applications of such a process in edible products, such as margarine, mayonnaise, etc., are apparent.
Thus, the value of available feed materials such as soybean oil, cottonseed oil, linseed oil, corn oil, peanut oil, sunflower oil, safflower oil, canola oil, olive oil, rich bran oil, sesame, and almond, etc., can be enhanced by processing to give fractions which are enriched or deleted in unsaturation. Other highly saturated feeds, such as tallow, lard, coconut, palm oil, butter fat, etc., may be reacted with unsaturated fatty acids via an interesterification process, as disclosed in U.S. Pat. Nos. 4,275,081 or 3,328,439, to increase the degree of unsaturation and the product thereof can be separated by the process of the invention.
The separation of many classes of compounds by selective adsorption on molecular sieves or zeolites as well as other adsorbents is well known. Also, various separations based on the degree of unsaturation are known, e.g., esters of saturated fatty acids from unsaturated fatty acids with X or Y zeolites exchanged with a selected cation, from U.S. Pat. No. 4,048,205, monoethanoid fatty acids from diethanoid fatty acids with cross-linked polystyrenes, e.g., "Amberlite", from U.S. Pat. No. 4,353,838. A process for separating a mixture of triglycerides, based on the iodine values, is shown in U.S. Pat. Nos. 4,277,412 and 4,284,580 in which permutite and surface-aluminated silica gel adsorbents, respectively, can be used. However, both of these require silver-exchanged surface-aluminated silica gel adsorbents, which is not only undesirable in food product preparation, but rapid fouling of these adsorbents by any impurities in the feed mixtures has limited commercial application of these materials. Ou U.S. Pat. No. 4,961,881 describes a process for overcoming the deactivation of the surface-treated silica gel of U.S. Pat. No. 4,284,580 by continuously or intermittently regenerating the adsorbent with hydrogen peroxide or an organic peroxide. The adsorbents of the present invention are stable and thus do not exhibit the rapid deactivation that is exhibited by the prior art absorbents.
The adsorption properties of silica gels have been reported and found useful in certain analytical separations, e.g., thin layer chromatography (TLC). For example, Plattner et al, Lipids 14 (2), (1979), pp 152-3 reported that triglycerols could be separated by both chain length and number of double bonds with reverse phase columns, i.e., .mu.-Bondapak C.sub.18 or .mu.-Porasil silica gels with octadecyl silyl groups bonded to silica particles. Also, Plattner et al, JAOCS 54 (11) (November 1977) pp 511-15. Acetonitrile: acetone (2:1 v/v) mixtures were used as elution agents. Neither, however, describes a process capable of separating triglycerides by degree of unsaturation in bulk quantities nor the preferred desorbents of the present invention.
Japanese Public Disclosure No. 192797/86 discloses a method for concentrating eicosapentaenoic acid and docosahexaenoic acid in their triglyceride forms with silica gel chemically bound with an octadecyl group or a styrene-divinylbenzene copolymer.
Zinnen U.S. Pat. No. 4,784,807 discloses the separation of triglycerides based on degree of unsaturation with omega zeolite or carbon adsorbents and ketones, toluene and ketone/aliphatic hydrocarbon mixtures as desorbents.
The invention herein can be practiced in fixed or moving adsorbent bed systems, but the preferred system for this separation is a countercurrent simulated moving bed system, such as described in Broughton U.S. Pat. No. 2,985,589, incorporated herein by reference. Cyclic advancement of the input and output streams can be accomplished by a manifolding system, which are also known, e.g., by rotary disc valves showing in U.S. Pat. Nos. 3,040,777 and 3,422,848. Equipment utilizing these principles are familiar, in sizes ranging from pilot plant scale (deRosset U.S. Pat. No. 3,706,812) to commercial scale in flow rates from a few cc per hour to many thousands of gallons per hour.
The functions and properties of adsorbents and desorbents in the chromatographic separation of liquid components are well known, but for reference thereto, Zinnen et al U.S. Pat. No. 4,770,819, which relates to the separation of diglycerides from triglycerides with omega zeolite or silica adsorbents, is incorporated herein. From FIG. 1 and Example II of this patent, it can be seen that Zinnen et al was not able to separate triglycerides on the basis of degree of unsaturation with silica gel, since all the triglycerides eluted at the same time.
I have found that monounsaturated triglycerides can be separated from mixtures with polyunsaturated triglycerides with silica gel which will selectively adsorb the more highly unsaturated triglycerides contained in triglyceride mixtures relative to monounsaturated triglyceride components, provided that the temperature at which the separation is conducted is at least about 100.degree. C. and the desorbent liquid contains a lower ketone (C.sub.3 -C.sub.8) in concentrations of from 5% (vol.) up to about 25% (vol.), preferably from about 10-20% (vol.) or p-cymene in concentrations of 20-100% (vol.).
Silica gel is thermally stable and thus can be regenerated easily at elevated temperatures without collapsing the pore structure. Furthermore, since there are no metal exchange ions, silica gel is deemed suitable for the separation of food products and is chemically stable to impurities contained in the feed.